The present invention relates to communication systems and more particularly to optical receivers in one embodiment.
Maximizing optical receiver sensitivity is important to improving optical communication link performance. In particular, optical receiver sensitivity contributes to transmission distance and maximum data rate. For a given transmission distance, modulation scheme, and data rate, increasing receiver sensitivity can increase the spacing between optical amplification sites. The challenge is to accurately recover data from weak signals.
For conventional optical communication links employing amplitude modulation, the typical optical receiver configuration incorporates a photodetector for recovering an electrical modulation signal from the received light and some type of threshold device for converting the recovered signal to a sequence of zeros and ones that reflect transmitted data. Increasingly, forward error correction (FEC) codes are used to improve link performance and thus the optical receiver may also incorporate a decoder.
In some respects, optical receivers do not differ in their basic architecture from certain receivers used in other transmission media such as wireless, copper, etc. One point of difference, however, concerns threshold operations. In other types of systems, the threshold will typically be set at a midpoint of the modulation envelope with received signal levels above the midpoint being treated as ones, for example, and received signal levels below the midpoint being treated as zeros.
This positioning of the threshold at midpoint, however, assumes that noise levels are independent of the transmitted data. In optical systems, however, noise levels are typically greater during periods when a one is transmitted than when a zero is transmitted. Furthermore, the extent of this effect varies depending on the particular characteristics of the optical communication link.
Thus, using a modulation envelope midpoint as the threshold leads to non-optimal receiver operation. Shifting the threshold somewhat lower to accommodate the greater expected noise on the one data leads to a greater likelihood of accurately recovering the transmitted data, thereby increasing sensitivity. But no one fixed threshold will be appropriate in every situation and in fact the ideal position of the threshold for maximum likelihood detection of transmitted data will vary somewhat over time.
In one prior art approach to implementing a receiver capable of varying its decision threshold, a duplicate photodetector and thresholding stage are provided. The received optical signals split between a primary receiver chain and the duplicate components. The threshold used in the duplicate receiver chain is varied to find a minimum error point. Then this threshold level is adopted by the primary receiver chain for use in recovering the transmitted data. One problem with this approach is that it requires duplication of expensive receiver components such as photodiodes. Another problem is that the optimal position of the threshold in fact depends in part on photodiode characteristics and these will vary between the primary receiver and the duplicate receiver such that a threshold found to be optimal for the duplicate receiver will not necessarily be optimal for the primary receiver.
What is needed are systems and methods for controlling optical receiver threshold to optimize recovery of transmitted data while not significantly increasing costs.